This invention concerns improved thin film photovoltaic cells and their preparation. In particular, this invention concerns an improved thin film photovoltaic cell having significantly increased adhesion between the semi-conducting and conducting layers. The increased adhesion is obtained by incorporating minor amounts of the conducting element in the semi-conducting layer.
Thin film photovoltaic cells prepared from elemental and compound semi-conductors are well known. For example, photovoltaic cells based on silicon, gallium arsenide, and cadmium sulfide have been prepared from single crystals. However, in order to economically obtain thin film polycrystalline photovoltaic cells from these semi-conductive materials, technical improvements in the fabrication process are still necessary. Accordingly, a great deal of research effort has been expended on their development and improvement. For instance, U.S. Pat. No. 2,820,841 granted to Carlson et al on Jan. 21, 1958 describes the fabrication of a typical thin film polycrystalline cadmium sulfide photovoltaic cell. In general, the cadmium sulfide cells comprise a substrate, a conducting electrode layer, a semi-conducting polycrystalline cadmium sulfide layer, a photovoltaic barrier layer containing a group IB metal, and a collecting electrode layer. The interfacial contact between the semi-conducting layer and the barrier layer forms a photovoltaic junction. It is believed that this junction is of the P-N type and that the mechanism of photovoltaic generation involves the formation of electron-hole pairs in the semi-conducting layer in response to the action of incident radiation having absorbable wavelengths. The charge carriers diffuse from the semi-conducting layer across the junction, creating a potential difference which in turn causes an electrical current to flow through an external circuit completed by the collecting and conducting electrodes.
Various deposition processes have been shown to be useful in forming semi-conducting thin films suitable for use in photovoltaic cells. Suitable processes include physical deposition, chemical vapor deposition, spray pyrolysis deposition, and electrodeposition. Physical deposition is carried out by collecting vaporized elemental or salt forms of the material to be deposited on a substrate to form a thin film. Chemical deposition uses a vapor phase reaction to deposit a thin film on a substrate. Spray pyrolysis is carried out by spraying a solution of the material to be deposited on a heated substrate. Electrodeposition relies on an electrochemical reaction in a solution of the material to be deposited to produce a thin film. Each of these deposition processes is well known. For instance, U.S. Pat. No. 3,880,633 granted to Jordan et al on Apr. 29, 1975 describes a method of continuously fabricating cadmium sulfide photovoltaic cells. According to this method, a conducting layer of tin oxide and a semi-conducting layer of cadmium sulfide are applied to a floating plate of glass by spray pyrolysis deposition. The conducting layer is formed by spraying a solution of tin chloride, reactants, and dopants on the heated glass substrate. A similar conducting layer can be obtained by substituting indium chloride for tin chloride. Similar conducting layers can be obtained by sputter deposition or by chemical vapor deposition. In the Jordan process, the semi-conducting layer is formed by spraying an aqueous solution of cadmium chloride and N,N-dimethylthiourea over the tin oxide layer.
Thus, in the usual arrangement of layers comprising typical thin film photovoltaic cells a glass or metallic substrate is first coated with a conducting layer. Then a polycrystalline semi-conducting layer is deposited. Regardless of the process used to deposit these layers, it is important that there is good adhesion between the conducting layer and the semi-conducting layer in order to maintain a low series resistance through the cell and to give the cell long-term thermal stability.
Accordingly, it would be advantageous to provide a method for improving the adhesion between the conducting and semi-conducting layers. Various materials, such as dopants, have been suggested for inclusion in the semi-conducting layer. Dopants are described in the art as materials which can be added to the semi-conducting layer to reduce the resistivity of the layer thereby increasing the efficiency of the cell. Materials comprising indium, gallium, and aluminum have been used as dopants to darken the cadmium sulfide semi-conducting layer making it more light absorbent in the red end of the spectrum. Preferably the dopant is applied to the semi-conducting layer. However, dopants can be applied simultaneously with the semi-conducting layer. It would be particularly advantageous to provide a method for improving the adhesion of the conducting and semi-conducting layer which also provides the advantages of a dopant.